1. Field of the Invention
This invention relates to range finding equipment, especially to laser-based range finding and distance measuring apparatus. More particularly, this invention relates to a measuring apparatus which may be held, aimed, and operated with one hand by a user to determine measurable parameters such as distance, elevation, inclination and range. This invention also relates to an electronic filtering feature in a laser-based range finding apparatus, wherein the electronic filter, when enabled by the user, rejects spurious reflected signals from the ranging calculation.
2. Description of the Related Art
A laser-based distance and ranging measuring device is described in U.S. Pat. No. 5,359,404 entitled "Laser-based Speed Measuring Device," the disclosure of which applicants expressly incorporate by reference herein. Other recently issued U.S. Patents relating to laser based measuring devices, assigned to the assignee of the present invention, include U.S. Pat. Nos. 5,612,779 and 5,652,651, the disclosures of which applicants also expressly incorporate by reference herein.
Distances, as measured in a laser-based ranging device, are generally determined by calculating the time of flight of a laser pulse to and from a desired target. An elapsed time measurement is calculated based on, among other things, a clock signal, a counter triggered on by transmission of a laser pulse and triggered off by the receipt of a reflected pulse signal from the target. The reflected pulse signal is detected by the ranging device using a light sensing receiver. The distance may then be determined based on the clock pulse width using the number of pulses counted between transmission and reception or by using methods such as the precision timing technique described in the above identified patents.
The accuracy of the desired ranging measurement in a laser range finding device can be compromised if reflective obstacles exist between the ranging device and the desired target. For instance, if the ranging device is being used to measure an outdoor distance between the ranging device and a wall, it is possible that the ranging device may detect reflected pulses from objects in the line of sight, such as reflections off of shrubbery and trees, etc. In prior ranging devices, it is understood that optical filters have been used to distinguish between noise signals and the desired signal reflected from the target. However, these filters are somewhat cumbersome to carry, install and remove. What is needed is a laser range finding device having a built in feature whereby the user of the device in the field can simply exclude such unwanted reflections. For example, what is needed is a means to selectively adjust the sensitivity of the receiver of the device, so that when the user attempts to measure a distance or range to an object where other reflective objects exist between and in the line of sight of the object and the ranging device, the ranging device will ignore the spurious reflections from the non-target objects.
In addition, because prior art devices can weight six pounds or more, such devices are preferably tripod mounted. An operator typically needs both hands to both hold and operate a conventional range finding device. When the device is operated while hand held, the operator soon fatigues, especially when the unit is operated at arms length. Thus, there is a continuing need for range finding and distance determining equipment which can be used without causing fatigue to the operator and which can be operated by a user in the field having only one free hand to hold, aim and operate the apparatus.
Thus there remains a need for a distance measuring apparatus which includes a simply selectable user actuated capability to discriminate between true target reflections and spurious reflected signals, and such equipment which may be used with one hand and without the necessity for the operator to continually need to obtain operational instructions from a manual or instruction book. It is against this background that the significant improvements and advancement of the present invention have taken place in the field of laser based distance measuring apparatuses.